The invention relates to a downhole drilling motor for use in subsurface well drilling operations.
Downhole drilling motors generally comprise a motor housing which is connected at the lower end of an elongate drill string and an output shaft which drives a rotary drill bit. The motors are usually hydraulic motors that are driven by the flow of drilling fluid through the drill string.
During drilling operations the weight of the drilling assembly is commonly utilized to exert an axial force to the bit. The magnitude of said axial force may fluctuate during drilling due to friction between the drill string and borehole wall and due to drill string dynamics. Conventional downhole drilling motors have a tendency to stall if a fluctuating axial force is exerted to the bit since such fluctuations generate similar fluctuations in bit torque.